The development of methods for the introduction of foreign genes into organisms has had a profound impact on fields of medicine and agriculture. While the movement of genes within plant species or between closely related plant species by traditional methods based on sexual reproduction has played an important role in crop improvement for most of this century, the pace of crop improvement by such methods has been slow and limiting due to the reliance on naturally occurring genes. Recent advances in the field of genetic engineering has led to the development of genetic transformation methods that allow the introduction of recombinant DNA, into organisms. The recombinant DNA methods which have been developed have greatly extended the sources from which genetic information can be obtained for crop improvement. Recently, new crop plant varieties, developed through recombinant DNA methods, have reached the marketplace. Genetically engineered soybeans, maize, canola and cotton are now widely utilized by North America farmers.
Rapid progress has been made in developing the tools for manipulating genetic information in plants. Plant genes are being cloned, genetic regulatory signals deciphered, and genes transferred from entirely unrelated organisms to confer new agriculturally useful traits to crop plants. Recombinant DNA methods significantly increase the gene pool available for crop improvement.
Maize or corn (Zea mays) is, on an economic basis, the most important crop grown in the United States. The continued success of American agricultural depends, to a large extent, on the continued success of U.S. maize producers. Certainly, a key factor that has lead to and helped maintain the preeminent position of maize in U.S. agriculture is the development of improved cultivars of maize. While maize geneticists and plant breeders have improved and will continue to improve maize through classical breeding approaches, molecular biologists have recently demonstrated that genetic engineering approaches may be employed to provide maize cultivars with new traits that were not attainable through classical breeding approaches. In only a few years since their initial release, commercial cultivars that have been genetically engineered for herbicide and insect resistance, have achieved phenomenal success.
While strides have been made in the genetic transformation of maize, a major difficulty in producing transgenic maize plants continues to be regenerating transformed maize cells into transformed maize plants. Thus, maize scientists have focused their efforts on transforming cells that have the greatest likelihood of being regenerated into a transformed plant. Maize scientists have utilized cells derived from maize embryos that have been subjected to culture conditions that are known to promote embryogenic-tissue formation. While such cells are amenable to transformation and regeneration, the recovery of transformed maize plants from a transformation attempt has been less than desirable. Methods employing cells from embryogenic-tissue cultures are both costly and laborious because such methods involve the development and maintenance of such cultures. Methods that involve the use of immature embryos themselves as the source of cells for transformation may be more desirable, particularly if the cells from the isolated embryos can be transformed soon after isolation. However, methods for transforming isolated, immature embryos have generally involved incubating the embryos after isolation for several days in culture under conditions which favor the formation of embryogenic tissue. Thus, improved methods for transforming maize cells and regenerating transformed maize plants are desired.